1. Field of the Invention
This invention relates to the decoration of sheeting and more particularly to the decoration of materials such as standard, lightweight, cellulosic sheets. This invention also relates to the embossment of plastic film and more particularly the high speed embossment of such films with holographic patterns.
2. Description of the Prior Art
Cellulosic sheets are normally decorated by imprinting. To achieve special effects, this requires special inks and relatively complex printing procedures. In addition, some decorative effects cannot be realized by imprinting. Complex procedures limit the speed at which decoration can take place.
In addition, it is difficult to achieve shallow or surface embossment for cellulose sheets because of their grainy texture.
There are wide-spread applications for embossed plastic films. One example is provided by plastic films embossed with holographic patterns. These are used in applications where security sensitive items such as holographic labels are used.
A present practice is to emboss the plastic film with holographic patterns at a relatively slow rate of travel, on the order of 25-30 feet per minute. This is done by direct embossing of polyester film before or after metallization. The film is a single-heated, and softened somewhat, in order to accept an embossing pattern. If the film is softened to the point where it readily accepts the pattern there can be significant distortion of the film and consequently of the pattern embossed upon it. This has resulted in the need to emboss at relatively low temperatures, i. e., below the true softening temperature of the film, slow speeds and relatively high pressures. Such films are typically embossed at rates under about 50 feet per minute.
Another procedure involves a large number of rolls disposed in series and preheated before embossment. It is necessary to use a large number of preheated rolls for careful control over the temperature and the accomplishment of embossment without distortion of the plastic film. In order to reach a desired compromise between web speed and temperature, the leader of the plastic film that is to be embossed is fed through a complex series of rolls. The operation is initially started at a slow speed which is programmed to increase to a desired speed consistent with the temperature at which suitable embossment should take place without damaging the underlying plastic film. Not only is this complex but the final speed that is achievable in practice is in the range from about 30-50 feet per minute.
In a further procedure a film of polyvinylchloride is extruded. The film still retains some of its residual heat when the embossment takes place, but this is awkward. If there is too much of a delay between the time of extrusion and the time of embossment, the pattern is relatively poor. Another objection is that an extruder must be tied to an embossing arrangement.
It is accordingly an object of the invention to provide an alternative and simplified technique for the decoration of sheeting. A related object is to achieve special effects without printing. Another related object is to achieve decorative effects that cannot be realized by ordinary imprinting.
It is an object of the invention to facilitate the embossment of plastic film, particularly films that are to have holographic or diffraction pattern images. A related object is to facilitate the production of security sensitive material such as holographic labels.
Another object is to increase the rate at which plastic films can be embossed without significant distortion. A related object is to avoid the need for embossment at relatively low temperatures, slow speeds and relatively high pressures.
A further object of the invention is to eliminate the need for a large number of preheated rolls in order to control film temperature and accomplish the desired embossment without plastic film distortion.
Another object of the invention is to eliminate the need for embossment following extrusion, with the embossment relying upon the residual heat of extrusion in order to form a suitable pattern. A related object is to improve the quality of patterns associated with embossment following extrusion. Another related object is to eliminate the need for operating embossing apparatus in conjunction with an extruder.
A further object of the invention is to realize desired decorative effects for cellulosic sheeting, at high speed, despite the grainy texture.